Optical signal transmission of data can have advantages over electrical transmission of data, particularly when data must be transmitted at very high rates. These advantages are usually associated with the wide bandwidth required in the transmission system, wherein transmission lines used for electrical data transmission are more subject to noise and signal attenuation than the optical fibers used for optical data transmission.
Light sources for such optical data systems can be directly or indirectly modulated by electrical signals that represent the transmitted data. Indirect modulation involves the use of an optical modulator that responds to the electrical signals that represent the data to be transmitted. Such optical modulators are typically of the electro-absorptive, electro-dispersive, or phase-shift type. Regardless of type, the optical modulator is usually configured to cause a variation or shift in the intensity of the optical beam from the light source in some relationship to the electrical signals.
Compact optical data transmitters of the indirectly modulated type preferably comprise a laser diode light source that is coupled to an electro-absorptive waveguide type modulator. Typically, the electro-absorptive modulator operates by utilizing the shift in transmissivity of the modulator's waveguide to a longer wavelength due to the application of a strong electric field.
This shift occurs with modulator waveguides of several different semiconductor structures, such as when the structure of the waveguide comprises a bulk semiconductor, a single isolated quantum well, multiple isolated quantum wells, or multiple coupled quantum wells (a superlattice). With this type of modulator, the wavelength at which the modulator's waveguide changes from relatively transmissive to relatively opaque changes as a function of the potential of the electrical input signals applied to it.
Thus, for any given instantaneous potential applied to the electrical input of the modulator, there is a range of wavelengths of light that may be passed through the modulator's waveguide with relatively low absorption, a range of wavelengths with relatively high absorption, and a narrow range of wavelengths at which the characteristics of the waveguide shift from relatively transmissive to relatively absorptive, an "electro-absorptive edge" region.
If an operating wavelength for the light source is selected so that the change in potential of the electrical input signals, plus any applied bias potential, shifts or varies the electro-absorptive edge about this operating wavelength, a modulated optical signal that has a relatively large depth of modulation is generated by the modulator. The modulator is biased to shift the wavelengths of the electro-absorptive edge with respect to the operating wavelength so that a relatively small change in input signal causes a relatively large change in absorption of the modulator.
Absorption modulators that have a semiconductor waveguide structure of the superlattice type exhibit very high contrast because of the high absorption to transmission ratio of light that is transmitted through this type of modulator using a small shift in modulator input signal potential. For instance, better than 30 db of contrast is achievable with as little as 3 volts shift in modulator input signal potential.
The optimal operating point in terms of contrast change as a function of modulation signal potential shift is in a region of relatively high absorption. To minimize insertion loss, a short modulation region is desirable, typically in the range of 100 to 200 microns in length. A short modulation region also improves the frequency response of the modulator.
One problem with such a modulator is that it has dimensions that make it difficult to handle and package. Another problem is that an external modulator of this type can induce optical feedback to a laser light source to which it is coupled. Variation in the feedback to a diode laser source during modulation can unstabilize the output of the diode laser and cause it to mode hop.